Since the development of off-axis holography in the 1960's, volume holography has been identified as a candidate for high density data storage. Theoretically, up to 10.sup.14 bits of information can be stored in 1 cm.sup.3 of a volume holographic medium. In addition, holographic storage promises fast data transfer rates, estimated at over 1 Gb/s. An underlying reason for the fast performance of holographic storage systems is that thousands of data bits are stored together in pages, rather than individually on a track. For information on holographic memory systems, see for example the articles by Heanue et al. in Science 265: 749-752 (1994), Hong et al. in Opt. Eng. 34(8): 2193-2203 (1995), and Psaltis and Mok in Scientific American 273(5): 70-78 (1995), or U.S. Pat. No. 4,927,200 (Hesselink et al.).
Briefly, in a typical volume holographic storage system data is stored in a photorefractive medium such as a lithium niobate (LiNbO.sub.3) or strontium barium niobate (SBN) crystal. The data is encoded as a page in a coherent signal beam that is allowed to interfere with a coherent reference beam within the recording medium; the interference pattern corresponding to a page is stored throughout the medium. For readout, only the reference beam is sent through the medium, and the interaction of the reference beam with the stored interference pattern produces a signal beam proportional to the beam originally used to store the pattern.
Several approaches have been used for multiplexing, or storage of multiple pages within a system. Typical approaches include angular, wavelength and phase-code multiplexing. One of the major concerns in mutiplexed holographic storage is the crosstalk between stored images. For a review of crosstalk considerations in holographic storage systems, see for example the article by Bashaw et al. in J. Opt. Soc. Am. B 11: 1820-1836 (1994), which is herein incorporated by reference.
In many data storage applications it may be desirable to prevent unauthorized access to the stored data. Secure data storage can be accomplished by encrypting the data itself, using known digital algorithms; a recovered data stream is then meaningless to a user without deciphering capability. In holographic systems it is also possible to encrypt data without processing the data itself. That is, secure storage of a data page can be achieved by encrypting the reference beam.
U.S. Pat. No. 3,711,177 describes encrypting data in a two-dimensional hologram on an ID card. Encryption is achieved by placing a random phase mask in the reference beam path. The same mask is then required for readout. Other patents discussing holographic encryption by phase-modulating the reference beam include U.S. Pat. Nos. 3,519,322 and 3,620,590, and UK patent 2,196,443B. The above-mentioned patents discuss encrypting single holograms, but do not address multiplexing or crosstalk considerations.
In an article in Sov. J. Qu. Elec. 7: 1147 (1977), Krasnov proposed random phase-code multiplexing, a technique wherein a different random phase mask is placed in the reference beam path for each hologram stored. This multiplexing technique allows secure data storage. However, the amount of data required to describe a large number of masks can be comparable to, or even exceed, the amount of data to be stored. Also, the reference beams in a random phase code system are in general not orthogonal, and the crosstalk performance of such a system is not optimal. The crosstalk problem is especially pronounced in random phase code systems using a small number of discrete elements comprising the mask, such as systems using phase spatial light modulators (PSLM).